Much effort has been made to increase the oxygen permeability of a contact lens to a level where the lens may be worn for extended periods of time without significant corneal swelling. One approach to increasing the oxygen permeability has been to include silicone monomers into materials used to form the contact lens. However, these silicone monomers tend to be large molecules, which may have many byproducts.
Various silicone containing monomers have found utility as starting materials in the production of medical devices, such as ophthalmic devices and particularly, soft contact lenses having improved permeability to oxygen. One class of suitable monomers includes tris and bis(trimethylsilyloxy)silylalkylglycerol methacrylates (“SiAGMA”). During the synthesis of SiAGMA certain diester acrylate impurities are also generated. The diester acrylates or free radical reactive difunctional groups act as crosslinkers during the polymerization of the SiAGMAs with other polymer forming components. Unfortunately, it can be difficult to control the amount of difunctional byproducts which are formed. The difunctional byproducts effect many of the properties of the lens, including modulus, equilibrium water content and elongation. Accordingly, the concentration of the difunctional byproducts must be controlled to ensure that the mechanical properties of the resulting medical device do not vary to an unacceptable degree.